The physical layer of NR (the 3GPP 5G mobile radio systems) is expected to handle a vast number of different transmission scenarios by supporting multiple transmission numerologies, variable data transmission time intervals and early decoding for latency critical applications. These new scenarios impose a need for the physical layer to be even more flexible than the case when LTE was first designed. In addition to these new transmission scenarios, the physical layer of NR should be able to handle different transmission characteristics in terms of large variations in SINR, Doppler, delay spreads and channel richness.
In mobile radio systems, reference signals for coherent demodulation of physical layer control and data channels signals may be transmitted within an OFDM waveform. The reference signal (RS) is multiplexed with the physical layer channels and mapped on the OFDM time-frequency resource grid as configured by the network. In LTE downlink, the demodulation can be based on either cell-specific RS (CRS) or UE-specific RS (e.g. DM-RS), the type of which to be used depends on configured transmission mode. The mapping of CRS on the time-frequency resource grid follows from the transmit-antenna configuration together with a cell-specific frequency shift, derived during initial access, whereas the DM-RS mapping depends on the number of MIMO-layers.
The UE-specifically configured DM-RS can be pre-coded in the same way as the corresponding physical layer channels and dynamically adapt the number of MIMO-layers to the radio channel conditions. Hence, one DM-RS antenna port is used per spatial MIMO layer scheduled to the UE. An antenna port is associated to a given RS pattern of resource elements over a time and frequency region. Different antenna ports may map to different resource elements to provide orthogonality.
Each transmitted MIMO layer thus has one associated DM-RS antenna port, and, since the data transmitted in that layer is precoded with the same precoder as the associated DM-RS, it is said that the data is transmitted on the associated DM-RS antenna port. The receiver will use the associated antenna port when demodulating the data of a layer.
It may be preferable if different antenna ports are orthogonal when transmitted, as it gives better channel estimation performance at the receiver. This can be achieved by separation in time and frequency (different resource elements) or by using a combination with orthogonal cover codes (OCC) across multiple resource elements in time or frequency.
With DM-RS, LTE supports up to 8-MIMO layers in downlink by using OCC in time.
FIG. 1 exemplarily illustrates the mapping of CRS and DM-RS patterns.
Like LTE, NR will be using OFDM based waveforms with reference signals and physical layer channels mapped on a time-frequency resource grid (in particular, for DL, in UL a special form of OFDM may be used, SC-FDM). Reference signals to be used in NR for demodulation of physical layer channels have not yet been specified but will primarily be based on UE-specifically configured DM-RS patterns that can support multiple transmission numerologies, variable data transmission time intervals and early decoding for latency critical applications.
FIG. 2 shows DM-RS structures that have been discussed to meet requirements of early decoding or for low Doppler/low UE mobility (e.g., low relative speed). In this structure, the early transmission of DM-RS enables demodulation and decoding of data to start almost directly after receiving the second OFDM symbol in the slot.